Neon lighting is formed of brightly glowing, electrified glass tubes or bulbs that contain rarefied neon or other gases. Neon lights are a type of cold cathode gas-discharge light. A neon tube is a sealed glass tube with a metal electrode at each end, filled with one of a number of gases at low pressure. A high potential of several thousand volts applied to the electrodes ionizes the gas in the tube, causing it to emit colored light. The color of the light depends on the gas in the tube. Neon lights were named for neon, a noble gas which gives off a popular orange light, but other gases and chemicals are used to produce other colors, such as hydrogen (red), helium (yellow), carbon dioxide (white), and mercury (blue). Neon tubes can be fabricated in curving artistic shapes, to form letters or pictures. They are mainly used to make dramatic, multicolored glowing signage for advertising, called neon signs, which were popular from the 1920s to the 1950s.
The range of outer diameters for the glass tubing used in neon lighting is 9 to 25 mm; with standard electrical equipment, the tubes can be as long as 30 metres (98 ft). The pressure of the gas inside is in the range 3-20 Torr (0.4-3 kPa), which corresponds to a partial vacuum in the tubing. Noble gases, known for being chemically unreactive, produce vivid colors. Neon, in particular, gives off a bright glow. The other noble gases, argon, helium, xenon, and krypton, are also used to create bright, colorful signs and displays. Radon, the other noble gas, is radioactive and not used in signs.
The neon sign is an evolution of the earlier Geissler tube, which is an electrified glass tube containing a “rarefied” gas (the gas pressure in the tube is well below atmospheric pressure). When a voltage is applied to electrodes inserted through the glass, an electrical glow discharge results. Geissler tubes were quite popular in the late 19th century, and the different colors they emitted were characteristics of the gases within. They were, however, unsuitable for general lighting; the pressure of the gas inside typically declined in use. The direct predecessor of neon tube lighting was the Moore tube, which used nitrogen or carbon dioxide as the luminous gas and a patented mechanism for maintaining pressure; Moore tubes were sold for commercial lighting for a number of years in the early 1900s. The discovery of neon in 1898 by the British scientists William Ramsay and Morris W. Travers included the observation of a brilliant red glow in Geissler tubes. Travers wrote, “the blaze of crimson light from the tube told its own story and was a sight to dwell upon and never forget.” Following neon's discovery, neon tubes were used as scientific instruments and novelties.
Neon tube signs are produced by the craft of bending glass tubing into shapes. A worker skilled in this craft is known as a glass bender, neon bender or tube bender. The neon tube is commonly made out of 4 or 5-foot long straight sticks of hollow glass sold by sign suppliers to neon shops worldwide, where they are manually assembled into individual custom designed and fabricated lamps.
Tubing in external diameters ranging from about 8-15 mm with a 1 mm wall thickness is most commonly used, although 6 mm tubing is now commercially available in colored glass tubes. The tube is heated in sections using several types of burners that are selected according to the amount of glass to be heated for each bend. These burners include ribbon, cannon, or crossfires, as well as a variety of gas torches. Ribbon burners are strips of fire that make the gradual bends, while crossfires are used to make sharp bends. The interior of the tubes may be coated with a thin phosphorescent powder coating, affixed to the interior wall of the tube by a binding material. The tube is filled with a purified gas mixture, and the gas ionized by a high voltage applied between the ends of the sealed tube through cold cathodes welded onto the ends. The color of the light emitted by the tube may be just that coming from the gas, or the light from the phosphor layer. Different phosphor-coated tubing sections may be butt welded together using glass working torches to form a single tube of varying colors, for effects such as a sign where each letter displays a different color letter within a single word.
“Neon” is used to denote the general type of lamp, but neon gas is only one of the types of tube gases principally used in commercial application. Pure neon gas is used to produce only about one-third of the colors (mostly shades of red and orange, and some warmer or more intense shades of pink). The greatest number of colors (including all shades of blue, yellow, green, violet, and white, as well as some cooler or softer shades of pink) produced by filling with another inert gas, argon, and a drop of mercury (Hg) which is added to the tube immediately after purification. Each gas used in neon lights has its own color. Neon is red, helium is orange, argon is lavender, krypton is gray or green, mercury vapor is light blue, and xenon is gray or blue. Mixing gases and elements added to a neon light creates different hues. When the tube is ionized by electrification, the mercury evaporates into mercury vapor, which fills the tube and produces strong ultraviolet light. The ultraviolet light thus produced excites the various phosphor coatings designed to produce different colors. Even though this class of neon tubes use no neon at all, they are still denoted as “neon.” Mercury-bearing lamps are a type of cold-cathode fluorescent lamps.
Each type of neon tubing produces two different possible colors, one with neon gas and the other with argon/mercury. Some “neon” tubes are made without phosphor coatings for some of the colors. Clear tubing filled with neon gas produces the ubiquitous yellowish orange color with the interior plasma column clearly visible, and is the cheapest and simplest tube to make. Traditional neon glasses in America over 20 years old are lead glass that are easy to soften in gas fires, but recent environmental and health concerns of the workers has prompted manufacturers to seek more environmentally safe special soft glass formulas. One of the vexing problems avoided this way is lead glass' tendency to burn into a black spot emitting lead fumes in a bending flame too rich in the fuel/oxygen mixture. Another traditional line of glasses was colored soda lime glasses coming in a myriad of glass color choices, which produce the highest quality, most hypnotically vibrant and saturated hues. Still more color choices are afforded in either coating, or not coating, these colored glasses with the various available exotic phosphors.